The major purpose of this investigation is to explore mechanisms of integration of SV40 viral DNA into the cellular chromosomal DNA leading to cellular transformation and oncogenesis. Our major objectives are: 1) To determine if integration can occur without causing transformation, 2) The structure of the integrated genome-specifically, do tandemly repeated viral sequences occur in transformed cells, 3) To use the formation of adeno-SV40 hybrid DNA molecules as a model for defining the parameters of recombination and the possible type of recombinant structures that can be formed, 4) Ultimately to relate the mode of integration with the expression of the integrated SV40 DNA sequences.